1. Field of the Invention
The present invention relates to the field of synthetic resins, and more particularly to thermoplastic polymers which are useful as binders in high-energy compositions.
2. Description of the Related Art
High energy formulations, such as those used in propellant, explosive and pyrotechnic (PEP) applications, often comprise solid particulates, such as fuel and oxidizer particulates, dispersed and immobilized throughout a binder matrix which includes an elastomeric polymer. Certain thermoplastic elastomers (TPEs) are desirable binders for PEP applications because they can be processed and reprocessed by melt-casting or extrusion at a temperature which is suitable for processing of energetic materials. This is in contrast to other types of elastomers, such as chemically cross-linked or aplastic elastomers, which must be processed by more complex methods.
Of particular interest are TPEs which are useful as binders for PEP applications and are also xe2x80x9cenergeticxe2x80x9d, that is, contribute to the energy content of the PEP material. Such binders allow the PEP material to be more energetic overall than a PEP material made with a non-energetic binder.
Very few TPEs are available which are both energetic and suitable for use as PEP binders. Moreover, those TPEs which are available are made from polymers which are not readily degradable. Degradable polymers would be extremely desirable at the end of the PEP material""s life cycle, as disposal of such materials can be a major problem. Moreover, a degradable TPE could allow recovery of the chemicals forming the polymer for recycling and reuse, farther simplifying disposal and saving money.
One example of an energetic TPE binder of the contemporary art can be seen in U.S. Pat. No. 4,483,978, to Manser, entitled ENERGETIC COPOLYMERS AND METHOD OF MAKING THE SAME, which describes polymers made from oxetane or tetrahydrofuran monomers having energetic substituents. However, the described thermoplastic elastomers contain exclusively ether linkages in the backbone, and therefore are not readily degradable.
Another example is seen in the U.S. Pat. No. 4,806,613, to Wardle, entitled METHOD OF PRODUCING. THERMOPLASTIC ELASTOMERS HAVING ALTERNATE CRYSTALLINE STRUCTURE FOR USE AS BINDERS IN HIGH-ENERGY COMPOSITIONS. This patent describes a method of producing a block copolymer of A-blocks and B-blocks, in which the A-blocks and B-blocks are polyethers derived from oxetane and/or tetrahydrofuran. The A-blocks are described as crystalline below about 60xc2x0 C., and the B-blocks are described as amorphous down to xe2x88x9220xc2x0 C. Azido or nitrato monomers can be used to provide energetic polymers. However, the described thermoplastic elastomers contain exclusively ether and urethane linkages in the backbone, and therefore are not readily degradable.
U.S. Pat. No. 5,783,302, to Bitler et al., entitled THERMOPLASTIC ELASTOMERS, describes thermoplastic elastomers which have A blocks or B blocks or both A and B blocks, which are crystalline. The crystallinity is attributed to the presence of crystallizable side chains. The patent does not describe degradable polymers or energetic polymers.
A paper by Cannizzo et al., Proceedings of the International Symposium on Energetic Materials Technology, American Defense Preparedness Association, Phoenix, Ariz., September 1995, also describes energetic TPEs, containing azido groups, which are suitable as PEP binders.
Based on our reading of the art, then, we have decided that what is needed is an energetic thermoplastic elastomer which is readily degradable for recovery of the building blocks of the elastomer.
It is therefore an object of the present invention to provide an improved energetic thermoplastic elastomer.
It is also an object of the invention to provide an improved energetic binder for propellant, explosive and pyrotechnic (PEP) applications.
It is a further object of the invention to provide energetic binders which can be readily broken down at the end of the product life cycle.
It is a yet further object of the invention to provide energetic binders which can be broken down by hydrolysis.
It is a still further object of the invention to provide energetic binders which can be broken down to allow recovery and recycling of the energetic components and other constituents of the binder, as well as other components of the pyrotechnic, explosive or propellant containing the binder.
These and other objects of the invention are accomplished by providing a hydrolyzable thermoplastic elastomer which may be used as a binder in PEP applications. The hydrolyzable thermoplastic elastomers of the present invention may be energetic.
Specifically, a hydrolyzable thermoplastic elastomer of the present invention contains a first polymer block and a second polymer block having a different repeating unit than the first polymer block. The first and second polymer blocks are bridged by a bridging unit which is an organic chain molecule. The connection between the bridging unit and a polymer block may have an ester linkage which allows hydrolyzability. The elastomer may also have an additional bridging unit between some of the polymer blocks linked via a urethane linkage.
In another embodiment, a hydrolyzable thermoplastic elastomer of the present invention contains a first polymer block and a second polymer block having a different repeating unit than the first polymer block. The first and second polymer blocks are bridged by a bridging unit which contains at least one formal (that is, geminal diether) linkage, and the connection between the bridging unit and a polymer block has a urethane linkage.
The first polymer and second polymer blocks may be polyethers such as polyoxetanes. They may be formed of monomers having energetic substituents such as xe2x80x94N3, xe2x80x94NO2, xe2x80x94ONO2, and xe2x80x94N(NO2)-alkyl for providing an energetic elastomeric product. Alternatively, the first or second polymer may also be a polymer such as poly(caprolactone)diol polyformal.
The present invention also encompasses methods of making hydrolyzable elastomers. One method of preparing a hydrolyzable thermoplastic elastomer includes the step of reacting a first polymer comprising a first repeating unit and having hydroxyl termini; a second polymer comprising a second repeating unit different from said first repeating unit and having hydroxyl termini; and a linear dicarboxylic acid; so as to yield a resultant polymer comprising ester linkages formed from the carboxylic acid groups of said dicarboxylic acid and the hydroxyl termini of said first and second polymers. The reaction may be performed by preparing a solution containing the first and second polymers and the dicarboxylic acid as well as dimethylaminopyridine and dicyclohexylcarbodiimide. In addition, dimethylaminopyridine hydrochloride may be added as a cocatalyst. The resultant polymer from the reaction may be further chain-extended by further reacting with an organic diisocyanate.
Here, the first and second polymers may be polyethers, such as poly(oxetanes) or poly(tetrahydrofurans), or one of the polymers may be poly(caprolactone)diol polyformal. The polymers may be formed of monomers having energetic substituents such as xe2x80x94N3, xe2x80x94NO2, xe2x80x94ONO2, and xe2x80x94N(NO2)-alkyl for providing an energetic elastomeric product. For example, the first polymer may be poly(AMMO) or poly(BAMO), where BAMO is 3,3-bis(azidomethyl)oxetane and AMMO is 3-azidomethyl-3-methyloxetane.
A second method of preparing a hydrolyzable thermoplastic elastomer, includes the steps of: end-capping the hydroxyl termini of a first polymer comprising a first repeating unit by reacting the first polymer with an organic diisocyanate; end-capping the hydroxyl termini of a second polymer comprising a second repeating unit different from said first repeating unit, by reacting the second polymer with an organic diisocyanate; and reacting the end-capped first polymer and the end-capped second polymer with a linear diol comprising a formal linkage, to form urethane links between said end-capped first and second polymers and said linear diol. As in the first method, the first polymer and second polymer blocks may be polyethers such as polyoxetanes. They may be formed of monomers having energetic substituents such as xe2x80x94N3, xe2x80x94NO2, xe2x80x94ONO2, and xe2x80x94N(NO2)-alkyl for providing an energetic elastomeric product.
The present invention provides new energetic thermoplastic elastomers which are copolymers containing at least two different polyether blocks linked by hydrolyzable linkages. The use of two different polyether blocks in the copolymer allows the design of TPEs having desirable melting characteristics.
A variety of polyether blocks may be used in the present invention. Of particular interest are polyethers based on substituted oxetane or tetrahydrofuran. The general structure of polyoxetanes is given in equation (1) and the general structure of substituted poly(tetrahydrofuran) is given in equation (2). 
Here, the substituents R can be selected from a variety of moieties, including: alkyl, azidoalkyl, xe2x80x94NO2, nitroalkyl, xe2x80x94N(NO2)-alkyl, alkenyl, alkynyl, xe2x80x94O-alkyl, xe2x80x94NH(alkyl), and xe2x80x94N(alkyl)2.
Of interest as energetic polyethers are the polymers of 3,3-bis(azidomethyl)oxetane (BAMO), shown in equation (3), and 3-azidomethyl-3-methyloxetane (AMMO), shown in equation (4). 
For these respective polyoxetanes, groups R and Rxe2x80x2 in equation (1) would both be xe2x80x94CH2xe2x80x94N3 or would be xe2x80x94CH2xe2x80x94N3 and xe2x80x94CH3.
Other energetic polyoxetanes which are suitable for the present invention include polymers of 3-azidomethyl-3-nitratomethyloxetane (AMNO); 3,3-bis(methylnitraminomethyl)oxetane (AMNAMO); 3,3-bis(methylnitratomethyl)oxetane (BMNAMO); 3,3-bis(nitratomethyl)oxetane (BNMO); 3-methylnitraminomethyl-3-methyloxetane (MNAMMO); and 3-nitratomethyl-3-methyloxetane (NMMO). Non-energetic oxetanes include 3,3-bis(acetoxymethyl)oxetane (BAOMO); 3,3-bis(chloromethyl)oxetane (BCMO); 3,3-bis(ethyoxymethyl)oxetane (BEMO); 3,3-bis(fluoromethyl)oxetane (BFMO); 3,3-bis(hydroxymethyl)oxetane (BHMO); 3,3-bis(iodomethyl)oxetane (BIMO); 3,3-bis(methoxyethoxymethyl)oxetane (BMEMO); 3,3-bis(methoxymethyl)oxetane (BMMO); 3-chloromethyl-3-methyloxetane (CMMO); 3-hydroxymethyl-3-methyloxetane (HMMO); 3-iodomethyl-3-methyloxetane (IMMO); and 3-octoxymethyl-3-methyloxetane (OMMO).
In polymers of the present invention, two blocks formed of different polyethers (or one block formed of a polyether and the other block of poly(caprolactone)diol polyformal) are linked by a connecting group having a hydrolyzable linkage. The hydrolyzable linkage of the present invention may be an ester linkage or may be a formal, or gem-diol, that is, a unit of general structure Rxe2x80x94Oxe2x80x94CH2xe2x80x94Oxe2x80x94R. Therefore, one general composition of the present invention contains at least two polyether units of different repeating unit structure, and contains a formal linkage located between the two polyether units. Another general composition of the present invention contains two polyether units of different repeating unit structure, and contains an ester linkage between the two polyether units.
The following specific Examples detail synthetic methods and chemical structures of compositions of the present invention. In the following examples, the molecular weights of the polymers were determined by gel permeation chromatography, using a Toyo Soda Micropak TSK 4000H column plus two TSK 3000H size exclusion columns, each 30 cmxc3x970.75 cm. A Waters Model 6000A delivery system, model U6K injector, Model 440 UV detector and Model R-401 IR detector were used. The molecular weight determinations were calibrated with polyethyleneglycol standards. Selected polymers were further characterized using differential scanning calorimetry and qualitative elastomeric tests.